In conventional safety systems in, for example, cars, air-bags are integrated as a part of the system. Air-bags are designed as a supplement to other safety devices, such as belts and deformation zones provided in the car.
An air-bag is a device which is rapidly inflated with gas in the event that a vehicle is involved in a collision, in order to reduce the damages on the persons/passengers travelling inside the vehicle.
However, in some situations, the use of an air-bag has turned out not to provide an increased safety for the passenger. Thus, if an un-belted child is seated on the passenger seat and the air-bag is activated during a collision, it has turned out that the activation of the air-bag causes the passenger more damage than would be the result if the air-bag had remained inactivated.
Furthermore, if the passenger is an adult it has turned out that the air-bag should be activated, regardless of whether or not the passenger wears a seat-belt or not. Thus, it is insufficient to sense if the passenger wears a seat-belt or not, the age of the passenger must also be determined for the air-bag system to provide maximum security.
Also, the International patent application WO 95/00368 discloses a device for detecting the presence of a person in a seat, for example in a vehicle. According to WO 95/00368, two electrodes are provided on opposite sides of the thorax of the seated person. The two electrodes together with the thorax forms a capacitor, where the thorax acts as the dielectric. When the person is breathing the characteristics of the capacitor changes, whereby the changing of the characteristics can be measured. The use of the device according to WO 95/00368 can make it possible to distinguish between different animal, such as between a human and a dog.
However, the device as described in WO 95/00368 has a number of drawbacks. Thus, the device does not enable to distinguish between a human child and an adult. Also, the capacitor used for deriving the measured signal has drawbacks. For example, if a person seated in a seat of a vehicle is wearing a jacket, the characteristics of the dielectric in the capacitor will change, possibly resulting in a malfunction of the detection system.
Thus, the is a problem of how to in a secure manner determine the age of a seated person, in particular of a person seated in a vehicle equipped with an air-bag system.